Acute GI bleeding, especially involving the lower GI tract, may be diagnosed and regionally localized scintigraphically. Two radiotracer options exist: 99mTc RBC's and 99mTc sulfur colloid. Sulfur colloid is used less-often because of its shorter effective half-life. However, its higher lesion-background ratio lends it some utility in clinically unstable patients with a need for rapid diagnosis. 99mTc RBC's have a long half-life that allows for delayed imaging, but a low lesion-background ratio, and will detect bleeding in the 0.05-0.1 ml/min range (angiography requires approximately 1 ml/min or more for diagnosis of GI bleed). Images are generally obtained in 1-minute frames over the first 60-90 minutes. If negative, delayed images can be obtained up to 24 hours after radiotracer injection.

Diagnosis of GI bleed is made when an area of focal radiotracer uptake meets the three following criteria:
1. pool conforms to bowel anatomy
2. activity increases with time
3. activity moves antegrade or retrograde with time

Common pitfalls include GI mucosal uptake of free 99mTc pertechnetate, and genitourinary uptake (pelvic/ectopic kidney, uterine or penile blush, ureter, and bladder). These pitfalls may complicate the interpretation of pelvic activity which conforms to the above diagnostic criteria. Obtaining a lateral, and other views as necessary, in all cases of focal, increasing, and even mobile pelvic uptake is of special importance to differentiate true rectal bleeding from genitourinary uptake.